Hugely important for agriculture, barely produced in the US at all, mostly imported from Canada.
If Canada retaliates by barring or limiting potash exports, it would be so harmful to the American agriculture sector that Trump would go ballistic.
> They can and will move to the US because of these tariffs.
That's a lot easier said than done, and as US policies might change at any time -- it's a coin-flip what'll be the case in four years -- they'll probably just decide to wait it out and let US consumers eat the tax.
Seriously, you might think it's easy to relocate, e.g., a car manufacturing plant, but it sure ain't...